As electrophotographic photoconductors applied to copiers, laser printers and the like, hitherto, inorganic photoconductors composed of selenium, zinc oxide, cadmium sulfide and the like, which had been most commonly used, but in present day, organic photoconductors (OPCs) have become most commonly used which are more advantageous in reducing burden on global environment, low cost performance and high degree of design freedom, than the inorganic photoconductors. Recently, organic photoconductors are utilized at levels approaching 100% of the total amount of production of electrophotographic photoconductors. The organic photoconductors are required to be converted from supply products (disposable products) to machine parts, in response to the recent growing awareness of environmental protection.
So far, various attempts have been made to impart high durability to organic photoconductors. In present days, forming a crosslinked resin film on a surface of a photoconductor (e.g. PTL 1) and forming a sol-gel cured film on a surface of a photoconductor (e.g. PTL 2) are, in particular, highly expected for next-generation electrophotographic photoconductors.
The former has an advantage in that flaws and cracks hardly occur even when a charge transporting component is blended therein, thereby reducing yield loss. Especially, radical polymerizable acrylic resins are excellent in toughness, and thus use of them is advantageous in easily obtaining a photoconductor excellent in photosensitivity. In these two methods of using a resin having a crosslinked structure, a coated film is formed from plural chemical bonds, and thus even when the coated film is subjected to stress and part of the chemical bonds is broken, this will not immediately lead to abrasion of the photoconductor.
In the meanwhile, developing toners for use in electrophotography are advantageous in ecological property in production and achieving higher image quality, and therefore, polymerized toners (spherically shaped toners) are becoming more commonly used.
The polymerizable toners (spherically shaped toners) are spherical-shaped toners having no angular portion and produced by a chemical method such as a suspension polymerization method, emulsion-aggregation polymerization method, ester elongation method, or dissolution-suspension method. Polymerized toners differ in shape depending upon the production method employed, and polymerized toners for use in image forming apparatuses are made to have slightly more irregular shape than spherical-shape toners in consideration of easy cleanability and the like. A typical average spherical degree of toners is from 0.95 to 0.99, and typical shape coefficients, i.e., SF-1, and SF-2 are from 110 to 140. Note that when the average spherical degree is 1.0 and the shape coefficients SF-1 and SF-2 are 100, it indicates that the toner has a complete sphere shape.
Since polymerized toner particles are uniform in shape, the amount of electric charge to be retained by the toner tends to be relatively uniform. In addition, a wax and the like (in an amount of 5% to 10%) are easily internally added. Therefore, polymerized toners hardly run over the edge of a latent electrostatic image and are excellent in developing property, image sharpness, resolution, gray-scale tone and in transfer efficiency. Besides, polymerized toners have many advantages. For example, it is unnecessary to use oil in transfer process of image. On the other hand, this type of toner has drawbacks in that it is difficult to clean smear of toner and it is necessary to increase the amount of external additives with tendencies of employing oil-less process. As a result, inconveniences take place, such as toner filming easily occurs on a surface of the photoconductor. There have been many studies made to solve the drawbacks, and lots of proposals have been made so far.
In order to establish the cleanability of a polymerized toner, generally, it is desired for a photoconductor to have a low coefficient of friction at its surface and be capable of sustaining the coefficient of friction even in repetitive use thereof. For example, it has been known that the cleanability of a polymerized toner can be improved by applying a solid lubricant, such as zinc stearate, to the surface of a photoconductor (see NPL 1).
When a solid lubricant such as zinc stearate is externally supplied onto a highly durable electrophotographic photoconductor on which surface the above-mentioned radical polymerizable crosslinked acrylic resin film is laminated, inconveniently, the solid lubricant may not be readily accepted by the photoconductor surface. Most of this type photoconductors have a smooth surface. Therefore, the problem with the acceptability is believed attributable to the smoothness of the photoconductor. To solve this problem, PTL 3 discloses a technique of stably supplying a lubricant material onto a photoconductor by forming the photoconductor surface to have a rough surface. Specifically, PTL 3 discussed that it is advantageous to set a surface roughness (Rz-JIS-1994) of a photoconductor to 0.4 μm to 1.0 μm and, as a measure, to add a filler into a surface layer of the photoconductor. It is also described that the advantageous point is to maintain a specific surface roughness of the photoconductor.
However, even if photoconductor surfaces have a same Rz value, a variety of rough surface configurations are present. For instance, surfaces of photoconductors sometimes have a same Rz value despite a profound difference in a distance of a concave portion from a convex portion (one concave-convex cycle length). For this reason, in some cases, there are ranks of acceptability of zinc stearate among photoconductors having a same Rz. In order to improve the acceptability of zinc stearate on the surface of a photoconductor, it is necessary to set special requirements other than Rz. The surface roughness of electrophotographic photoconductors is an important item of properties, and in most cases, the surface roughness has been determined so far by a method defined in JIS B0601 etc., as in the case just disclosed in PTL 3.
As methods for measuring the surface roughness widely used, there are an arithmetic average roughness (Ra), a maximum height (Rmax) and a 10-point average roughness (Rz), and the like. However, these evaluation methods have a drawback that measured values vary when exceedingly concave and/or convex portions are present in the area of a photoconductor surface measured.
There have been no methods for accurately evaluating the degree of surface roughness, and then studies are made on parameters indicating the degree of surface roughness. The following describes the study on the parameters.
In PTL 4, over a cross-section curve (1) which is obtained by measuring a surface configuration with a surface roughens measurement device, a divided width (X) which is set in a center of an average line (2) is defined, and a surface roughness is evaluated by the number of peak units (4) formed of a pair of a top and a bottom adjacent to one another positioned beyond the divided width (X) per unit length (L). An organic photoconductor is produced using a base material in which the number of peak units (4), when the divided width (X) is set to 20 μm and the unit length (L) is set to 1 cam, is 100 or less.
In PTL 5, in order to solve a problem that cleaning defects tends to occur when a toner having a smaller diameter is used in view to forming high quality images, a cleaning roller to which a bias voltage is applied so as to separate charged toner from a photoconductor used, is provided upstream a cleaning blade and the photoconductor is designed to have a 10-point average surface roughness Rz of 0.1 μm to 2.5 μm.
Meanwhile, PTL 6 proposes a method for satisfying relationships of ΔT>Rz and 0 nm<ΔT+Rz<5 nm, where a depletion amount of film thickness per K-Cycle is defined as ΔT and a surface roughness is defined as Rz.
Further, PTL 7 discloses a system including a blade, a toner composition and a unused image forming member, in which the unused image forming member includes a surface on which a latent image is formed using the toner composition, and the surface of the unused image forming member has a surface roughness defined by the following relationships.R/ann4>KB(1−σ2)/32πEt2af andR/ann2<√{square root over ( )}⅜π2·(1+μ2)/μ·KB/┌·t/af·θ  (A)
In the relationships (A), R denotes an average height of convex portions in the surface, “ann” denotes a half (½) of the closest distance between adjacent convex portions on the surface, KB denotes a modulus of volume elasticity of the blade, σ denotes a Poisson's ratio of the toner composition, E denotes a Young's modulus of the toner composition, t denotes an average thickness of flat particles in the toner composition, “af” denotes an average radius of the flat particles, u denotes an average value between a toner-blade frictional coefficient and a toner-surface frictional coefficient, Γ denotes a Dupre work of adhesion between the surface and the flat particles, and A denotes a blade tip angle.
Further, PTL 8 proposes a cylindrically shaped electrophotographic photoconductor including a cylindrically shaped support and an organic photosensitive layer provided on the cylindrically shaped support, in which a circumferential surface of the electrophotographic photoconductor has a plurality of dimple concave portions; the circumferential surface has a 10-point average roughness Rzjis (A) of 0.3 μm to 2.5 μm when measured along a circumferential direction of the circumferential surface and has a 10-point average roughness Rzjis (B) of 0.3 μm to 2.5 μm when measured along a bus line direction of the circumferential surface; an average interval RSm (C) between concave portions and convex portions is 5 μm to 120 μm when measured along a circumferential direction of the circumferential surface of the electrophotographic photoconductor; an average interval RSm(D) between concave portions and convex portions is 5 μm to 120 μm when measured along a bus line direction of the circumferential surface; a ratio (D/C) of the average interval RSm (D) to the average interval RSm (C) is 0.5 to 1.5; the longest diameter of the dimple concave portions is ranging from 1 μm to 50 μm; and the number of dimple concave portions having a depth of 0.1 μm to 2.5 μm is 5 to 50 per 10,000 μm2 of the circumferential surface of the electrophotographic photoconductor.
It is also specified that the 10-point average roughness Rzjis (A) is preferably 0.4 μm to 2.0 μm, the 10-point average roughness Rzjis (B) is preferably 0.4 μm to 2.0 μm, the average interval RSm (C) between concave portions and convex portions is preferably 10 μm to 100 μm, the average interval RSm (D) between concave portions and convex portions is preferably 10 μm to 100 μm and the ratio (D/C) of the average interval RSm (D) to the average interval RSm (C) is preferably 0.8 to 1.2.
Furthermore, it is specified that a maximum height Rp (F) of the circumferential surface of the electrophotographic photoconductor is preferably 0.6 μm or lower, and a ratio (E/F) of a maximum depth Rv (E) of the circumferential surface to the maximum height Rp(F) is preferably 1.2 or greater.
PTL 9 discloses an electrophotographic photoconductor including a support and an organic photosensitive layer provided on the support, in which a plurality of dimple concave portions are formed on a surface of a surface layer of the electrophotographic photoconductor, the longest diameter of the dimple concave portions is ranging from 1 μm to 50 μm, the number of dimple concave portions having a depth of 0.1 μm or more and a volume of 1 μm3 or more is 5 to 50 per 100 μm square of the surface of the surface layer of the electrophotographic photoconductor, and a plurality of concave portions corresponding to the dimple concave portions formed on the surface of the surface layer are provided at a boundary surface between the surface layer and a layer provided immediately under the surface layer.
PTL 10 proposes an image forming apparatus including a plurality of image bearing members each having a conductive support and a photosensitive layer on the conductive support and each configured that a surface thereof is exposed to light so as to form a latent electrostatic image, a plurality of developing devices each provided corresponding to the plurality of image bearing members and each configured to develop the latent electrostatic image using a developer, and a plurality of cleaning units each provided corresponding to the plurality of image bearing members and each configured to rub against a surface of each of the image bearing members so as to remove the developer, wherein at least a pair of developer devices among the plurality of developing devices house developers which are same in color but different in brightness, and wherein a 10-point average roughness Rz of the surface of each of the image bearing members at an initial stage is controlled according to the brightness of the developers housed in the developing devices corresponding the each of the image bearing members.
PTL 11 proposes an image forming apparatus configured to form an image using an electrophotographic photoconductor which has such a surface roughness that a 10-point average surface roughness Rz is 0.1 μm to 1.5 μm or a maximum height Rz is 2.5 μm or lower and which has such a surface property that a friction resistance Rf, which is a tensile load measured when a polyurethane-made flat belt having a JIS-A hardness of 70 degrees to 80 degrees, a width of 5 mm, a length of 325 mm, a thickness of 2 mm and a self weight of 4.58 g is applied under a load of 100 g, a contact length in a circumferential direction is set to 3 mm and a contact area is set to 15 mm2, satisfies a relationship of 45 gf<Rf<200 gf.
PTL 12 proposes an image forming method which includes developing a latent image formed on an electrophotographic photoconductor using a developer; primarily transferring a toner image, which has been formed in a visible image by the developer, onto an intermediate transfer member; secondarily transferring the toner image, which has been transferred onto the intermediate transfer member, onto a recording material; and removing a residual toner remaining on the electrophotographic photoconductor after transfer of the toner image onto the recording material, wherein a surface roughness Ra of the electrophotographic photoconductor is 0.02 μm to 0.1 μm, a surface roughness Rz of the intermediate transfer member is 0.4 μm to 2.0 μm, and an energy reducing agent is supplied to a surface of the electrophotographic photoconductor, so that an image is formed.
PTL 13 proposes an image forming apparatus including an organic photoconductor, wherein in the organic photoconductor, an average value of concave-convex cycles of concaves and convexes provided in its surface is 10 times or more the volume average particle diameter of a toner used.
PTL 14 proposes an electrophotographic apparatus including an electrophotographic photoconductor which rotates at a circumferential speed of 200 mm/sec and a cleaning unit, wherein the electrophotographic photoconductor has a conductive support, a photosensitive layer and a surface protective layer, the photosensitive layer and surface protective layer being provided over the conductive support, wherein the surface protective layer contains a fluorine-containing resin particle in an amount of 35.0% by mass to 45.0% by mass relative to the total mass of the surface protective layer, wherein the electrophotographic photoconductor has a 10-point average roughness of 0.1 μm to 5.0 μm, a surface hardness of 0.1 to 10.0 when measured by Taber abrasion resistance test and a surface frictional coefficient of 0.1 to 0.7; wherein the cleaning unit is a rubber elastic blade, a linear pressure of the cleaning blade against the electrophotographic photoconductor is 0.294N to 0.441N/cm, a glass transition temperature (Tg) of a toner used is 40° C. to 55° C., a tensile elastic modulus (Young's modulus) of the cleaning blade is 784N to 980N/cm2, a rebound resilience of the cleaning blade is 35% to 55%, and a base surface of the cleaning blade contains a fluororesin fine particle.
PTL 15 proposes an image forming method using an image forming member which satisfies a relationship of d/t×0.01≦Ra≦0.5 when a relationship between a flatness (d/t) of a toner (d: volume average diameter, t: thickness of toner particle) and a surface roughness of the image forming member is represented by a center line average roughness Ra (μm).
Also, PTL 16, PTL 17, and PTL 18 each propose an image forming apparatus, in which concave and convex portions are provided in an image forming member, the concave and convex portions having a size smaller than the volume average particle diameter of a spherical-shaped toner used therein.
PTL 19 discloses an electrophotographic photoconductor including an electrophotographic photoconductor which rotates at a circumferential speed of 200 mm/sec and a cleaning unit, wherein the electrophotographic photoconductor has a conductive support, a photosensitive layer and a surface protective layer, the photosensitive layer and surface protective layer being provided over the conductive support, wherein the surface protective layer contains a fluorine-containing resin particle in an amount of 15.0% by mass to 40.0% by mass relative to the total mass of the surface protective layer, wherein the electrophotographic photoconductor has a 10-point average roughness of 0.1 μm to 5.0 μm, a surface hardness of 0.1 to 20.0 when measured by Taber abrasion resistance test and a surface frictional coefficient of 0.001 to 1.2.
Meanwhile, as methods for evaluating a surface configuration of a photoconductor, many evaluation methods using Fourier transform have been proposed (see PTL 20, PTL 21, PTL 22, PTL 23, PTL 24, PTL 25, PTL 26, PTL 27, PTL 28, and PTL 29). In the Fourier transform of these proposals, changes that frequently occur in signals can be grasped as a distribution of frequency components thereof, however, these evaluation methods are not advantageous in examining changes of signals that do not often occur. Also, from the result of the Fourier transform, inconveniently, where that change occurs cannot be detected because positional (time) information of a horizontal axis is completely lost after transformation.
Also, PTL 30 propose a method of evaluating a surface roughness of a base material, in which a cross-section curve of the surface of the base material is determined with a length of 100 μm from an arbitrarily selected position thereof in an axial direction of the base material by a method defined in JIS B0601, a position of the cross-section curve in a vertical direction thereof at the position spaced at regular intervals in the horizontal axis direction is measured, a distribution defined in JIS Z8101 at this point is found, a measurement value selected from surface roughness values of Ra, Rz and Ry which are defined in JIS B0601 is determined, and the surface roughness is evaluated using the distribution and the measurement value.
PTL 31 proposes a method of evaluating a surface state of an image forming apparatus component, in which a cross-section curve defined in JIS B0601 is determined, data arrays on positions spaced at regular intervals on the cross-section curve in a surface roughness direction is subjected to a multiresolution analysis, and the surface roughness is evaluated based on at least the result of the multiresolution analysis.
Furthermore, PTL 32 discusses a base material for electrophotographic photoconductor, which is evaluated for a state of a surface of an image forming apparatus component, by a method where a cross-section curve defined in JIS B0601 is determined, data arrays on positions spaced at regular intervals on the cross-section curve in a surface roughness direction is subjected to a multiresolution analysis, the surface roughness of the image forming apparatus component is evaluated based on at least the result of the multiresolution analysis.
Even with any of the above methods for evaluating a surface roughness, there is a problem that the cleanability of electrophotographic apparatuses using a small-diameter toner or polymerized toner cannot be accurately evaluated. That is, with an evaluation method using surface roughness values Ra, Rmax, Rz and the like, a surface roughness cannot be accurately grasped. For this reason, a method has been employed so far in which in measurement of a surface roughness, first, a recording chart obtained by a surface roughness/profile measuring device is preliminarily saves, and then a surface roughness is examined from a cut wave form recorded in the recording chart, but there is a need to read the tendency of the recording chart, which requires a specific skill and some experience.
As having been described above, conventional methods for evaluation a surface roughness (a center-line surface roughness Ra, Rmax, Rz) have a drawback that the cleanability of a photoconductor in an electrophotographic apparatus using a small diameter toner or polymerized toner cannot be accurately evaluated.
Also, PTL 3 has the following drawbacks. In an Example thereof, an alumina fine particle is used. Alumina fine particles are unstable in terms of dispersibility of filler in a coating liquid, and thus some contrivance is necessary to determine film forming requirements. In another Example using a polymethylsilsesquioxane fine particle, it cannot be said that the acceptability of lubricant on a surface of a photoconductor is not sufficient. It is conceivable that the photoconductor cannot satisfactorily bear a solid lubricant on its surface due to large size concaves and convexes on the surface of the photoconductor.
A crosslinked-resin-surface-layer coating liquid has a low viscosity because it is mainly formed of a monomer component. By contrast, a silicon-containing fine particle such as a silica fine particle, and a silicone resin fine particle, has usually high dispersion stability in a crosslinked-resin-surface-layer coating liquid, and thus the use thereof is especially advantageous in terms of production, among a variety of fillers. However, inconveniently, conventional techniques have the following difficulties.
In PTL 33, in Example 2 described in paragraph [0162] and subsequent part, a silicon-containing fine particle is used. It is however cannot be said that the acceptability of solid lubricant on the surface of a photoconductor is sufficient. It is conceivable that the photoconductor cannot satisfactorily bear the solid lubricant on its surface due to exceedingly large concaves and convexes provided therein. There is a need to add a new different technique thereto.
PTL 34 discussed that an inorganic fine particle (hydrophobized silica) having an average particle diameter of 0.05 μm to 0.5 μm is dispersed in a thickness of 0.05 μm to 15 μm on a photosensitive layer having a surface roughness of 0.1 μm to 0.5 μm, which has been formed on a conductive support having a surface roughness of 0.01 μm to 2 μm. It is described that this method can achieve high durability of a photoconductor and prevent a reduction in resolution due to adhesion of contamination such as corona products on a photoconductor surface by subjecting the silica particle to a hydrophobization treatment. By effect of the hydrophobization of the inorganic fine particle, repellency of water-droplet (due to a wide contact angle) can be exhibited, however, it is impossible to prevent adhesion of corona products, and so image flow cannot be prevented. To solve the problem, for example, as can be seen in PTL 35, occurrence of image flow is avoided by using alumina as a filler. However, as described above, in a case of a crosslinked resin surface layer, it is difficult to directly use alumina in the coating liquid because of the problems described above.
Further, PTL 36 discusses that a lubricant removing unit to electrostatically remove a powder-form lubricant remaining on an image bearing member is provided in non-contact with the image bearing member.
In an image forming apparatus in which a solid lubricant is externally added to a surface of a photoconductor, the acceptability of solid lubricant on the photoconductor affects the abrasion rate of the photoconductor surface and the cleanability of toner and influence the quality of print images. In present, a technique for satisfactorily improving the acceptability of solid lubricant on a photoconductor surface in which a highly durable crosslinked resin surface layer is laminated has not yet been obtained.
As having been described, on providing of high durability to electrophotographic photoconductors, drastic improvements can be expected by forming a crosslinked resin film on the photoconductors surfaces. The cleanability of polymerized toners, which can be said as most frequently used for developers, is an important subject of technique. To solve the subject, application of solid lubricant to a surface of a photoconductor is advantageous. However, electrophotographic photoconductors with a crosslinked resin film being provided at the uppermost surface thereof are poor in coatability of solid lubricant, and therefore it has been unable to fully use their excellent durability.